Reusable high temperature thermal protection system

ABSTRACT

A reusable phase change material (PCM) heat shield is disclosed. The heat shield comprises: a thermally conductive casing, PCM, thermally conductive open cell foam, and heat pipes. The heat flows through the casing and open cell foam into the PCM, heating it up. The PCM changes phase twice, from solid to liquid. During the solid liquid phase change, heat pipes begin to draw heat away from the PCM to a secondary location that re-radiates the heat away. The open cell foam serves to help channel hear into the PCM. In one embodiment, the PCM heat shield can be used for thermal protection of an atmospheric entry vehicle (ARV). In another, the PCM heat shield may be applied to an aircraft engine to transfer extracted heat to preheat incoming air. In another, the PCM heat shield is integrated into the structure of a spacecraft, and used to both carry loads and protect against high temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

Under 35 USC 119(e), this application claims the benefit of U.S. Patent Application Ser. No. 61/347,759, entitled “REUSABLE HIGH TEMPERATURE THERMAL PROTECTION SYSTEM,” filed on May 24, 2010, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to heat shields and more particularly relates to a thermal protection method and system for heat shields.

BACKGROUND OF THE INVENTION

Heat shields are used in the aerospace industry for vehicles and engines that operate in high temperature environments. The state of the art in aerospace heat shields is radiative protection. Utilizing a material that has extreme high temperature resistance, high thermal conductivity, and high emissivity, this material is applied to the leading edge or nosecone of a vehicle. The heat that is generated there gets transferred into the material, and then conducted up the vehicle, before being radiated out at a cooler section of the vehicle. The state of the art has three main problems that have prevented its use on all vehicles and engines: weight, brittleness, and manufacturability. The materials that have the high temperature resistance, usually some compound of hafnium or zirconium, are all very dense. Radiative heat shields require acreage coverage, having a continuous shield from the bottom to top of a vehicle to work properly. Large coverage with a dense material forces the heat shield to be very heavy. These materials are also very brittle, meaning that they cannot be used to help support the loads generated by the vehicle or engine. Last, also due to the material brittleness, manufacturing smooth shapes out of the material is difficult. Accordingly what is desired is a system and method that addresses the above identified issues. The system and method should be easy to implement, cost effective, and adaptable to existing environments. The present invention addresses such a need.

Accordingly, what is desired is to provide a system and method that overcomes the above issues. The present invention addresses such a need.

SUMMARY OF THE INVENTION

A reusable phase change material (PCM) heat shield is disclosed. The heat shield comprises: a thermally conductive casing, PCM, thermally conductive open cell foam, and heat pipes. The heat flows through the casing and open cell foam into the PCM, heating it up. The PCM changes phase twice, from solid to liquid. During the solid liquid phase change, heat pipes begin to draw heat away from the PCM to a secondary location that re-radiates the heat away. The open cell foam serves to help channel heat into the PCM.

In one embodiment, the PCM heat shield can be used for thermal protection of an atmospheric entry vehicle (ARV). In another, the PCM heat shield may be applied to an aircraft engine to transfer extracted heat to preheat incoming air. In another, the PCM heat shield is integrated into the structure of a spacecraft, and used to both carry loads and protect against high temperatures.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the six components of the heat shield.

FIG. 2 illustrates how the heat shield diffuses incoming heat.

FIG. 3 shows how the heat is removed from the system.

FIG. 4 shows one variation of the system, designed for use on atmospheric reentry vehicles (ARV's).

FIG. 5 shows how the nylon layer is created.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates generally to heat shields and more particularly relates to a thermal protection method and system for heat shields. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

The objective of the present invention is to provide lighter, more robust, more manufacturable heat protection for use in high temperature environments solving the problems described above.

To achieve the above objective, the invention pertains to both the heat shield and method of heat shield operation. The present invention is a heat diffusion and storage device used for thermal protection. It protects by storing and channeling heat to other parts of the system. The casing and inlaid foam channels heat into a phase change material. The phase change material lowers the system temperature to allow the use of more common materials to radiate heat out. The following figures further explain how the device functions.

FIG. 1 shows the six components of the heat shield: a metallic casing such as a titanium beta 21S alloy casing 100, nylon coating on the inside of the casing 101, aluminum open cell metal foam 104, heat pipes 105, and attachment standoffs 106. The thermally conductive casing 100 and foam 104 transfer incoming heat into the nylon 102.

FIG. 2 illustrates how the heat shield diffuses incoming heat. The heat is absorbed by the nylon 101, allowing the casing 100 to be exposed to high heat fluxes for limited periods of time. The nylon 101 absorbs heat by heating up as a solid, then changing phase. The heat absorbed during the phase change caps the maximum temperature approximately to the solid-liquid temperature.

FIG. 3 shows how the heat is removed from the system. Once the nylon begins to change phase to a liquid, a bank of heat pipes 105 inlaid in the nylon activate to take heat away. The liquid solidifies as the heat is transferred to a cooler part of the system for eventual removal to the environment, such as by radiation into space. This cycle allows the full thermal utility of the nylon to be used, minimizing dead weight. It also lowers the system temperature, allowing the metal casing 100 to retain enough strength to carry structural loads. These two aspects help this heat shield design be lighter than the state of the art.

FIG. 4 shows one variation of the system, designed for use on atmospheric reentry vehicles (ARV's). It utilizes the vehicle trajectory dynamics to maximize heat transfer. All ARV's undergo moderate to high accelerations during the entry. In this heat shield design, the accelerations are used to both to stratify liquid/solid mixtures. The degree of heating faced by an ATV is proportional to the magnitude of acceleration experienced. In this variation of the system, due to the presence of an oxidizing environment, the system temperature is kept below a specified limit, to minimize the oxidation rate of the casing. Metal is used as the casing material 100 to increase both system impact resistance and manufacturability. Also, in this variation, if the radiation heat transfer stops, or reverses, the system can mitigate effects for a time by decomposing all of the nylon 103. As the nylon 103 decomposes, pressure will be created in the casing, and after a specified pressure is reached, relief valves 106 located on the side of the system will evacuate the decomposed nylon, dumping the heat from the system. This additional safety makes this heat shield design more robust, allowing it to be put through punishing environments without needing extensive material checks.

During assembly of the device, special attention must be given to the contact resistance. In this design, no gap must be present between the casing, nylon, water ice, and metal foam.

FIG. 5 shows how the nylon layer 102 is created. To ensure there are no gaps, the nylon 102 must be poured into the casing as a liquid, and then be allowed to solidify. Once solid, the nylon 102 must go through at least three phase change cycles under applied pressure to ensure there are no voids inside the solid. Once the nylon 102 is in-place, the heat pipes 105 and aluminum metal foam 104 is inlaid into the titanium beta 21S metal casing 100.

Thus, following these steps, contact resistance can be minimized, ensuring no hot spots form anywhere in the system.

The size of the aluminum 7075 metal foam 104 pores is also crucial. The pores must be large enough to allow nylon to flow through without being blocked by surface tension, and small enough to ensure the nylon is heated evenly. The pores in the foam 104 must also be open, to allow liquids and gasses to flow through the foam 104. The foam 104 is shaped to form fit into the casing 100, to allow easy fitting when the system is cool.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. 

1. A heat shield comprises: a thermally conductive casing; a phase change material (PCM) coupled to the casing; thermally conductive open cell foam coupled to the PCM; and a plurality of heat pipes coupled to the foam, wherein the heat flows through the casing and open cell foam into the PCM, wherein the PCM changes phase twice, from solid to liquid, during the solid liquid phase change the heat pipes begin to draw heat away from the PCM to a secondary location that re-radiates the heat. 